ABSTRACT Humoral immunity is dependent on the expression of immunoglobulin (Ig) to fend off pathogenic challenges. The humoral immune system has evolved to produce Ig with a broad repertoire of binding specificities. Class switch recombination (CSR) is used to attain diversity of Ig effector function and tissue localization. The murine IgH constant region locus is organized: 5'-V(D)J-C[unreadable]-C?-C?3-C?1-C?2b-C?2a-C?-C?-3'. CSR involves an intra-chromosomal deletional rearrangement that focuses on regions of repetitive switch (S) DNA located upstream of each CH gene (with the exception of C?). The process of CSR can be thought of as composed of three phases including, initiation, S/S synapsis and resolution and repair. AID induced DNA lesions at S regions initiates the process. I propose to examine events leading to S/S synapsis, and discern chromatin modifications associated with transcription and DNA repair. Using the chromosome conformation capture technique (3C), my laboratory has newly investigated the long- range interactions between the [unreadable] intronic enhancer (E[unreadable]) located between the VH and CH genes and the 3'E? enhancer located at the 3'-end of the IgH locus together with the various GLT promoters. We find that in B cells, the E[unreadable] and 3[unreadable]E? enhancers are in close spatial proximity forming an unique chromosomal loop configuration. B cell activation leads to recruitment of the germline transcript (GLT) promoters to the E[unreadable]:3[unreadable]E? complex in a cytokine dependent fashion. This structure facilitates S/S synapsis since S[unreadable] is proximal to E[unreadable] and the downstream S region are co-recruited with the targeted GLT promoter to the E[unreadable]:3[unreadable]E? complex. We propose that GLT promoter association with the E[unreadable]:3[unreadable]E? complex creates an architectural scaffolding that promotes S/S synapsis during CSR and these interactions are dependent on the stabilizing influence of AID. Chromatin remodeling is an important regulatory mechanism controlling the accessibility of S DNA to AID. We have defined histone modifications differentially found in the S and C regions. Our studies indicate chromatin accessibility is correlated with increased histone acetylation and H3K4me3 at the S regions whereas reduced accessibility is associated with hypoAc and the H3K36me3 mark downstream of the S region. We will study the causual relationship between accessibility and these histone modification. NARRATIVE Humoral immunity is dependent on the expression of immunoglobulin (Ig) to fend off pathogenic challenges. The humoral immune system has evolved to produce Ig with a broad repertoire of binding specificities. We study the molecular processes by which new types of Ig are expressed.